1. Field of the Invention
The present invention relates to a method of manufacturing a silicon film having crystallinity or a film having crystallinity and containing silicon. The present invention disclosed in the present specification can be used, for example, in manufacturing a thin film transistor (called a TFT).
2. Description of the Related Art
A thin film transistor (hereinafter referred to a TFT, etc.) using a thin film semiconductor is known. This is constituted by forming a thin film semiconductor, especially, a silicon semiconductor film on a substrate, and using this thin film semiconductor. The TFT is used for various kinds of integrated circuit, and especially has attracted attention as a switching element provided for each pixel of an active matrix type liquid crystal display device, or as a driver element formed in a peripheral circuit portion. Moreover, the TFT has also attracted attention as an indispensable art for a multilayer structure integrated circuit (solid IC).
It is simple to use an amorphous silicon film as a silicon film used for the TFT. However, there is a problem that the electrical characteristics thereof is far lower than those of a single crystal semiconductor used for a semiconductor integrated circuit. Thus, the amorphous silicon film has been employed for only limited uses such as a switching element of an active matrix circuit. The characteristics of the TFT can be improved by using a silicon thin film having crystallinity.
A silicon film having crystallinity other than single crystal silicon is referred to as polycrystalline silicon, polysilicon, microcrystalline silicon, or the like. Such a silicon film having crystallinity can be obtained by first forming an amorphous silicon film, and then crystallizing the amorphous silicon film through heating (thermal annealing). This method is referred to as a solid phase growth method since the amorphous state is transformed into the crystalline state while the film maintains the solid state.
However, in the solid phase growth of silicon, heat temperature of 600xc2x0 C. or more, and time of 10 hours or more are required, so that there is a problem that it is difficult to use an inexpensive glass substrate as a substrate. For example, corning glass 7059 has a glass distortion point of 593xc2x0 C., so that there is a problem in carrying out thermal annealing at a temperature not lower than 600xc2x0 C. when consideration is given to enlarging an area of the substrate.
As to such a problem, according to the study by the present inventors, it has been proved that when a small amount of metal element of some kind, such as nickel and palladium, is deposited on the surface of an amorphous silicon film, and then heating is carried out, crystallization can be made under such conditions that a temperature is 550xc2x0 C. and a processing time is about 4 hours. Of course, when annealing is carried out at a temperature of 600xc2x0 C. for about 4 hours, a silicon film having more excellent crystallinity can be obtained (Japanese Patent Unexamined Publication No. Hei 6-244103).
The above-mentioned small amount of element (metal element for promoting crystallization) may be introduced by a method of depositing a coating film of the metal element or a compound thereof by a sputtering method (Japanese Patent Unexamined Publication No. Hei 6-244104), by a method of forming a coating film of the metal element or a compound thereof through a means such as a spin coating (Japanese Patent Unexamined Publication No. Hei 7-130652), by a method of forming a coating film by decomposing a gas containing the metal element through a means such as pyrolysis and plasma decomposition (Japanese Patent Unexamined Publication No. Hei 7-335548), or the like. Those methods may be changed according to the respective features.
Moreover, it is also possible to selectively adding the metal element into a specific portion and then to extend the crystal growth from the portion where the metal element has been added to the surrounding by heating (lateral growth method or side growth method). Since the crystalline silicon obtained by such a method has directionality in crystallization, the silicon shows extremely excellent properties in accordance with the directionality.
It is also effective to further improve the crystallinity by irradiation of intense light such as a laser beam after the crystallization step using the metal element (Japanese Patent Unexamined Publication No. Hei 7-307286). In the above-mentioned lateral growth method, it is also effective to carry out thermal oxidation subsequent to the lateral growth (Japanese Patent Unexamined Publication No. Hei 7-66425).
When crystallization was carried out using the metal element as described above, a more excellent crystalline silicon film was obtained under conditions of lower temperature and shorter time. Although temperature at heat treatment greatly depends on the kind of the amorphous silicon film, the temperature of 450 to 650xc2x0 C., especially 550 to 600xc2x0 C. was preferable.
However, the most serious problem of this method is the removal of the metal element. It can not be neglected for the metal element added into the silicon film to give bad influence to electrical characteristics and reliability. Especially, in the step of crystallization using the metal element, in the mechanism, since the metal element as mainly conductive silicide remains in a coating film, the metal element becomes a terrible cause for defects.
It is generally known that a metal element (especially, nickel, palladium, platinum, copper, silver, and gold) can be captured by a crystal defect, phosphorus, etc. For example, Japanese Patent Unexamined Publication No. Hei 8-330602 discloses a technique in which a phosphorus ion is implanted into a silicon film using a gate electrode as a mask, then the metal element contained in the silicon film is moved to a source and a drain region by carrying out thermal annealing (furnace annealing) or light annealing (laser annealing, etc.), and then the metal element is fixed (gettered) to reduce the concentration of the metal element in a channel formation region.
In Japanese Patent Unexamined Publication No. Hei 8-330602, when phosphorus is implanted into the source and drain regions, since a silicon film is made amorphous and crystal defects increase, the metal element can be gettered by phosphorus and the crystal defects. Here, phosphorus can be implanted not only into the source and drain regions but also into any portion as long as the portion is not a place where a channel formation region is to be provided. It is obvious for a skilled person that the metal element can be removed by the above method although the degree of removal is different according to the distance from the portion where phosphorus has been implanted.
In order to carry out gettering, it is necessary to carry out annealing for a sufficient time so that the metal element can move to a region where phosphorus has been implanted. Thus, thermal annealing is preferable for the purpose. However, annealing temperature effective for the gettering (although the temperature depends on the kind of the metal element) is generally more than 600xc2x0 C. When a process at such high temperature is carried out for a long time, the possibility of deforming a substrate As raised to cause the slippage of a mask (the misalignment of a mask) in a subsequent step of photolithography.
Thus, although the light annealing is preferable, Japanese Patent Unexamined Publication No. Hei 8-330602 does not particularly discuss a light source for the light annealing, and merely states that an excimer laser is used in an example. However, a pulse width of the excimer laser is not larger than 100 ns, and it is experimentally proved that gettering can not be sufficiently carried out by light irradiation for such a short time.
Japanese Patent Unexamined Publication No. Hei 8-330602 discloses that the substrate is irradiated with a laser beam from a place above the substrate. However, in any examples, since aluminum having high optical reflectivity is used and the thickness thereof is not less than 3,000 xc3x85, it is difficult to give a sufficient amount of heat to a channel formation region so that the metal element is moved.
The present invention has been made in view of the above described problems, and an object thereof is to provide conditions suitable for light annealing to thereby provide a method effective to remove a catalytic element.
The basic concept of the present invention is to heat a region where a metal element is to be removed, by light annealing, for a sufficient time and to a sufficient temperature. With respect to the heating for the sufficient time, a well-known rapid thermal annealing (RTA) method is preferable.
When the RTA is used, high gettering efficiency can be obtained by heating for one second to ten minutes, though the time also depends on temperature. Further, according to this method, only a specific material can be heated without directly heating a substrate.
Further, this heating step has not only the gettering function but also an effect of improving the crystallinity.
A crystalline silicon film obtained by using a metal element for promoting crystallization of silicon is in a polycrystalline state. When the RTA is carried out, the number of dangling bonds existing in grain boundaries is lessened and the grain boundaries are inactivated. This is effective in improving the characteristics of a device element in the case where a device is formed. That is, rearrangement of silicon atoms existing in the vicinity of the crystal grain boundaries is promoted, so that combination among silicon atoms in the crystal grain boundaries is promoted. As a result, the inactivation of the crystal grain boundaries is progressed.
According to the method of Japanese Patent Unexamined Publication No. Hei 8-330602, as described before, although a region where phosphorus has been implanted is sufficiently heated by light annealing (laser annealing), an important region where a catalytic element is to be removed, is not sufficiently heated. However, even if a gate electrode, which blocks out the light, is removed for the purpose of solving the problem, an essential solution can not be obtained.
The reason is as follows. That is, since a region where phosphorus has been implanted is amorphous, the region has light absorptivity higher than a crystalline region where the metal element is to be removed, the temperature of the portion where phosphorus has been implanted is higher than the temperature of the portion where the metal element is to be removed, and the amount of the metal element moving from the former portion to the latter portion can not be neglected as compared with the amount of the metal element moving from the latter portion to the former portion, so that the efficiency of gettering is lowered.
Of course, since the former portion includes a large amount of phosphorus and defects for capturing the metal element, most of the metal elements are fixed to those. However, some of the metal elements can move and the ratio is increased as the temperature is raised.
That is, if the temperature of the region where phosphorus has been implanted, is not lower than the temperature of the region where the metal element is to be removed, a sufficient effect can not be obtained.
In the present invention disclosed in the present specification, an energy is effectively absorbed in a mask material in the step of carrying out the RTA so that a region where gettering is to be carried out (that is, a semiconductor region covered by the mask), is selectively heated to a high temperature.
By intentionally making such a state, the movement of the metal element in the region where the metal element is to be removed, becomes active due to the high temperature, so that more metal elements flow into the region a temperature of which is lower and in which the phosphorus has been implanted, and are fixed.
At this time, since the region where phosphorus has been implanted, has a lower temperature, the movement of the metal element is suppressed and the metal element is gettered more effectively.
In order to realize the above concept, according to the present invention, a method of manufacturing a semiconductor device comprises the steps of selectively masking a part of a crystalline silicon film or a crystalline film containing silicon, which has been obtained by using a metal element for promoting crystallization of silicon; accelerating and implanting an element in group 15 into a region which has not been masked in the masking step; and radiating intense light to heat a masked region of the film at a temperature higher than other portions to move the metal element from the masked region of the film to the other portions, and is characterized in that a material used in the masking step has a property to absorb the intense light at absorptivity higher than the crystalline silicon film or the crystalline film containing silicon.
Further, according to another aspect of the present invention, a method of manufacturing a semiconductor device comprises the steps of selectively masking a part of a crystalline silicon film or a crystalline film containing silicon, which has been obtained by using a metal element for promoting crystallization of silicon; accelerating and implanting an element in group 15 into a region which has not been masked in the masking step; and radiating intense light to heat a masked region of the film at a temperature higher than other portions, and is characterized in that a material used in the masking step has a property to absorb the intense light at absorptivity higher than the crystalline silicon film or the crystalline film containing silicon.
Further, the present invention has the following steps:
(1) a step of crystallizing an amorphous silicon film by using a metal element;
(2) selectively forming a mask including a material, which has light absorptivity to light to be radiated in subsequent step (4) and has heat resistance, on a crystalline silicon film obtained in step (1);
(3) implanting phosphorus into the silicon film by using the mask; and
(4) carrying out an RTA process to the silicon film and the mask.
On the other hand, phosphorus may be selectively implanted by using a mask formed when the metal element is selectively introduced. This is another invention disclosed in the present specification, and has the following steps.
(1) selectively forming a mask including a material, which has light absorptivity to light to be radiated in subsequent step (5) and has heat resistance, on an amorphous silicon film;
(2) selectively introducing a metal element into the amorphous silicon film by using the mask, or forming a coating film including the metal element;
(3) heating the amorphous silicon film to crystallize the film;
(4) implanting phosphorus into the silicon film by using the mask; and
(5) carrying out an RTA process to the silicon film and the mask.
The important feature of the present invention disclosed in the present specification is the selection of the material of the mask. The thickness of the material is also important.
It is preferable to use tungsten, chromium, molybdenum, or titanium, which is superior in absorption of near infrared rays and visible rays, as the material. Since it is not preferable that these materials are brought into direct contact with the silicon film, it is preferable to provide a coating film having excellent barrier properties (for example, silicon nitride) between the material and the silicon film. Especially, in the above-mentioned another invention, since the step of thermal annealing for crystallization exists after the formation of the mask, a sufficient countermeasure is required so that the material of the mask is not dispersed into the silicon film at the thermal annealing step.
It is preferable that the thickness of the mask is not less than 1,000 xc3x85. If the thickness is too thin, the light absorptivity is insufficient. When the radiation of light is carried out from a place above the substrate in the RTA step, if the thickness of the mask is too thick, heat conduction is insufficient. Thus, it is preferable that the thickness is not larger than 5,000 xc3x85. Similarly, if the barrier film provided between the mask and the silicon film is too thick, the heat conductivity is not excellent. Thus, it is preferable that the thickness of the barrier film is not larger than 2,000 xc3x85. A thin (10 to 100 xc3x85) film of silicon oxide or the like may be provided between the barrier film and the silicon film for the purpose of increasing adhesiveness.
On the other hand, when the RTA is carried out by radiating the light from a place under the substrate (rear side), the thickness of the mask is not problematic as long as the mask sufficiently absorbs light, so that the thickness up to 1 xcexcm is possible. However, if the thickness is too thick, since absorbed heat is used for heating of the mask rather than heating of the silicon film, it is not preferable. Also in this case, the same is true of the thickness of the barrier film.
In the present invention, the temperature of the portion where the metal element is to be removed, is made 600 to 1,200xc2x0 C., preferably 700 to 1,000xc2x0 C. Since the portion absorbing light is concentrically heated in the RTA method, the temperature of the substrate itself is far lower than the above temperature. Thus, it is possible to neglect the influence by the RTA method to the substrate.
A kind or plural kinds of element selected from the group consisting of Fe, Co, Ni, Ru, Rh, Pd, Os, Ir, Pt, Cu, and Au may be used as the metal element. Especially, using Ni is most preferable in the reproducibility and effects.
Using phosphorus as the element in group 15 used for gettering is preferable. Especially, the combination of nickel and phosphorus is most preferable.
Phosphorus and nickel includes a number of combined states such as Ni3P, Ni5P2, Ni2P, Ni3P2, Ni2P3, NiP2, and NiP3.
Thus, in the case where nickel is adopted as the metal element for promoting crystallization and phosphorus is adopted for the element in group 15, it is possible to effectively capture the nickel as a combination with phosphorus. That is, gettering can be effectively carried out.
Other than phosphorus, the element in group 15 such as N, As, Sb, and Bi may be used.
In the gettering, a grain boundary in the silicon film becomes an obstacle to the movement of the metal element. In general, in the silicon film after solid phase growth, the metal element as silicide precipitates in the grain boundary, and as a result, the grain boundary grows. Since such silicide is thermodynamically stable (after all, the metal element precipitates in the grain boundary since the state is thermodynamically stable), the metal element is hard to move from this portion. Further, there is caused a problem to capture the metal element moved from other portion and fix the metal element.
On the other hand, when a laser annealing process is carried out by irradiating the silicon film, which has been crystallized in the solid phase growth, with pulse laser beams, a tendency for the metal element to precipitate in the grain boundary, is greatly decreased. This is because a processing time by the pulse laser annealing (especially with a pulse width of not larger than 1 xcexcsec) is too short to make a thermodynamically stable state. The growth of grain boundary is also insufficient. That is, in the silicon film after the pulse laser annealing, many metal elements exist in such a manner that the elements are dispersed in the silicon film. Thus, these metal elements are extremely apt to move, and there are also few large grain boundaries to capture the metal elements, so that gettering can be effectively carried out.
It is preferable to make the concentration of phosphorus higher than the concentration of the metal element by one figure or more. The concentration is preferably made a high concentration such as 5xc3x971019 to 2xc3x971021 atoms/cm3. When phosphorus is implanted, hydrogen, oxygen, nitrogen, or carbon with a concentration of 1xc3x971019 to 1xc3x971021 atoms/cm3 may be implanted at the same time. When a large number of these elements exist, crystallization at the RTA is hindered, so that the quantity of defects in the portion where phosphorus has been implanted, can be maintained. When the concentration of carbon, nitrogen, or oxygen is high, the transparency of the silicon film is raised, so that it is possible to lower the light absorption by the portion where phosphorus has been implanted, and to suppress heating of the portion.
The present invention is different from Japanese Patent Unexamined Publication No. Hei 8-330602 in that gettering is carried out in the step of defining an active layer of a transistor by etching of a silicon film. However, although a portion of a region where phosphorus has been implanted for the purpose of gettering, may be completely removed, the region can also be used as a part or the entire of source and drain regions of a transistor. If the region is planned to be used as a part or the entire of the source and drain regions of a P-channel transistor, it is sufficient to implant a p-type impurity (boron, antimony, arsenic, etc.) with a concentration exceeding the above-mentioned amount of implanted phosphorus. An example in which the region is used as the entire of the source and drain regions of the P-channel transistor, is disclosed in Japanese Patent Unexamined Publication No. Hei 8-330602.